It is generally known that, in a mobile communication system, each mobile station communicates with others via a subscriber switching node of a current location area. When the mobile station roams from the current location area to another area, the communication channel passing through the subscriber switching node in the current location area should be changed over to another communication channel passing through a subscriber switching node in the location area which is the roaming destination. The change of communication channels is executed by so-called handover control. The handover control adopted for conventional mobile communication systems will be described below.
FIG. 9 shows an example configuration of a mobile communication network in which a fixed terminal 5 and a mobile station 1 communicate with each other. In FIG. 9, all the nodes in the fixed network side are omitted with exception of the fixed terminal. The fixed network and the mobile communication network are connected via a POI(i.e., point of interconnection) in the way of the interface point. A gateway conducting & switching node 51, being connected with other networks via the POI 4, is further connected with subordinate conducting & switching nodes 41 and 42. The conducting & switching node 41 is further connected with a subscriber switching node 31 controlling a location area 21. The conducting & switching node 42 is further connected with a subscriber switching node 32 controlling another location area 22.
The nodes 31, 32, 41, 42 and 51 and the POI 4 are previously appended with identifications so called routing numbers RN. The routing numbers are assumed to be RN1 to RN6. Furthermore, the nodes and the POI 4 are connected via channels 61 to 66. The nodes 31, 32, 41, 42 and 51 and the POI 4 stores routing tables, respectively, as shown in following Table-1.
TABLE 1 ______________________________________ Routing tables node 31 node 32 node 41 node 42 node 51 POI 4 ______________________________________ RN1: -- RN1: 62 RN1: 61 RN1: 64 RN1: 63 RN1: 66 RN2: 61 RN2: -- RN2: 64 RN2: 62 RN2: 65 RN2: 66 RN3: 61 RN3: 62 RN3: -- RN3: 64 RN3: 63 RN3: 66 RN4: 61 RN4: 62 RN4: 64 RN4: -- RN4: 65 RN4: 66 RN5: 61 RN5: 62 RN5: 63 RN5: 65 RN5: -- RN5: 66 RN6: 61 RN6: 62 RN6: 63 RN6: 66 RN6: 66 RN6: -- ______________________________________
The routing tables in Table-1 show the correspondence between nodes in the routing destinations and the channels to be selected. The uses of the tables will be described more specifically. First, if a call connection request for the fixed terminal 5 is originated from the mobile station 1, the call connection request is judged, in the subscriber switching node 31, to be forwarded to the POI 4. Consequently, in the subscriber switching node 31, the line "RN6" in the corresponding routing table is referred. The numeral "61" is stored there, so that the call connection request is further forwarded to the conducting & switching node 41 via the channel 61.
Then, in the conducting & switching node 41, the line "RN6" in the routing table stored in the node 41 is referred. The numeral "63" is stored there, so that the call connection request is further forwarded to the gateway conducting & switching node 51 via the channel 63. Similarly, the line "RN6" is referred in the gateway conducting & switching node 51. The numeral "66" is stored there, so that the call connection request is forwarded via the channel 66. As such, the call connection request originated from mobile station 1 is forwarded to the POI 4 via the channels 61, 63 and 66. Then, if the fixed terminal 5 is in communicatable condition, call set up and communication are executed between the mobile station 1 and the fixed terminal 5.
FIG. 10 shows an example of channel connection, executed by the handover control, between switching nodes in the mobile communication network. In FIG. 10, it is assumed that the mobile station 1 roams from the location area 21 to the adjacent location area 22, so that a handover control is required in order to change the subscriber switching node for controlling the mobile station. In this case, the subscriber switching node 31 controlling the location area 21 specifies, by means of conventional processes, a subscriber switching node (e.g. the subscriber switching node 32 in FIG. 10) which controls the location area 22 which is the handover destination.
Then, the subscriber switching node 31 keeps the connection of channel from the gateway conducting & switching node 51 to subscriber switching node 31, namely, the channel 63 between the gateway conducting & switching node 51 and conducting & switching node 41 and the channel 61 between the conducting & switching node 41 and the subscriber switching node 31, while setting up a channel from the subscriber switching node 31 to the subscriber switching node 32 which is the handover destination. Then, when the channel assignment is completed, the channels toward the subscriber switching node 32, namely, the channel 61 between the subscriber switching node 31 and the conducting & switching node 41, the channel 64 between the conducting & switching node 41 and the conducting & switching node 42, and the channel 62 between the conducting & switching node 42 and the subscriber switching node 32 are connected. The handover control described above enables the mobile station 1 to communicate via the subscriber switching node 32 of the location area 22 which is the roaming destination.
As described above, in the conventional mobile communication system, when the handover control due to the roaming of mobile station 1 is executed, the channel from the gateway conducting & switching node to the subscriber switching node (the channel between the switching nodes 51 & 41 and the channel between the switching nodes 41 & 31 in the example of FIG. 10) is left connected, while a routing is executed from the subscriber switching node in which the handover originated to the subscriber switching node in the location area which is the roaming destination. Therefore, in view of the entire mobile communication network, application of the conventional technique is problematical, to the mobile station 1, an inefficient channel passing through the subscriber switching node 31, even though the channel being the optimum route, the route consists of the channel between the switching nodes 51 & 42 and the channel between the switching node 42 & 32, is existing. Furthermore, in addition to the circumstance described above, the handover control may be executed if mobile stations communicate with each other in a common mobile communication network and if the location area of either one is changed due to roaming. In this case, a problem quite similar to above will occur by the handover control.